The interventionist liberals of the Obama administration were a doleful bunch last week. It was the 20th anniversary of the siege of Sarajevo, when a Bosnian Serb army battered a city full of civilians with artillery while the United States issued ineffective cries of alarm. The comparison with this year's massacres in Syria was painfully apt.

Now, as then, the United Nations Security Council has asked both sides to stop shooting, to no great effect. Now, as then, the United States and its allies are rejecting the idea of military intervention as too difficult, too risky, too likely to add to the violence instead of ending it.

In Bosnia, it took the United States more than three years and many massacres to decide that diplomatic measures and sanctions weren't enough. But finally, in August 1995, then-President Clinton ordered airstrikes against the Bosnian Serbs; that turned the tide of the war and led to peace negotiations within weeks.

But there's one big difference between the situation in Bosnia and that in Syria: This time, the clock is moving faster.

Although the Obama administration still hopes to avoid military intervention in Syria and is publicly backing a U.N. effort to broker a cease-fire this week, it has also stepped deliberately onto a slippery slope that is likely to lead to more intervention.

Unlike with Bosnia, where the United States and its allies initially sought to be neutral in a civil war, this time the U.S. has already chosen a side: It has called on Syria's dictator, Bashar Assad, to step down, and it has embraced the opposition Syrian National Council.

At a meeting in Istanbul last week, Secretary of State Hillary Clinton announced an escalation of U.S. aid to the opposition. In public, she pointed to a doubling of medical and other humanitarian aid, plus the provision of communication equipment. Less publicly, officials confirmed that the new package also includes "non-lethal" help that will go to the Free Syrian Army, the newly formed opposition armed forces, including night-vision goggles and U.S. intelligence information such as early warnings of Syrian troop movements.

And while the United States has decided not to provide weapons to the rebels, it isn't objecting to military funding or arms shipments from Saudi Arabia, Qatar and other Arab states that would like to see Assad fall.

In the short term, the administration says it still hopes former U.N. Secretary General Kofi Annan can arrange a cease-fire, and that Assad will — improbably — decide to step down.

But I couldn't find anyone in the administration last week who believed that outcome was likely. For one thing, Assad believes he's winning; there's no reason for him to surrender now. The best hope seems to be that the government crackdown will become less lethal.

If the pace of the killing slows, that could buy time: time for economic sanctions to undermine the regime, time to cajole Russia to switch sides and help pull the rug out from Assad, but also time for the opposition and its new army to organize themselves into a more effective force.

If those measures fail to bring Assad down, the administration appears divided on how quickly to move toward military intervention. The Pentagon is reluctant to get involved in another war, as the Pentagon usually is. Obama's national security adviser, Tom Donilon, has also weighed in against any post-Libya temptation to "militarize" another problem. Clinton's State Department has sounded the most hawkish notes — in part, perhaps, because it's Clinton who has delivered most of the administration's public declarations that Assad must go.

But even the administration's humanitarian hawks don't think the moment for U.S. or NATO military intervention has arrived yet.

They'd like the U.N. Security Council to give its blessing first, or — if Russia and China continue to resist — at least NATO. They'd like the Syrian opposition to be better organized, with more assurance that military aid wouldn't fall into the hands of radical Islamists. They'd like Turkey to establish safe havens for the opposition along its border with Syria.

Eventually, though, the question of military intervention will change from if to when. The United States is already a little bit pregnant — already committed to helping Assad fall. It's merely looking for the least violent, lowest cost way to get there.

In Bosnia, it took more than three years for the United States to overcome its reservations and resort to military force. But that was a generation ago, when the idea of humanitarian intervention in a civil war was still novel.